Modeling a Seagrass Restoration Target for an Urbanized Florida Estuary

Rod A. Braun, Senior Environmental Analyst Palm Beach County Environmental Resources Management

Abstract Loss of seagrasses has been documented wo rldwide and has been correlated with anthropogenic eutrophication and alteration of habitats. Deve loping a seagrass restoration target or numeric goal based on available understanding of the history and physical characteristics of an estuary sets up guide lines for ongoing and future habitat restoration projects. Palm Beach County (Florida) De partment of Environmental Resources Management has developed a methodology to mode l a seagrass restorati on target in Lake Worth Lagoon. Potential sea grass habitat is modeled using a bat hymetric survey to develop a digital elevation model, s eagrass mapping from aerial photography to determine seagrass distribution by depth, and es tuarine substrate characteristics. A user can determine where seagrass restoration or recr uitment is likely to occur and can assist in setting a restoration target based on th ese factors. Additionally, the information developed provides an effective management tool to determine where restorative and protection efforts should be focused.

Summary

The objectives of this paper are to: (1) di scuss existing information used for the development of a seagrass restoration targ et for the Lake Worth Lagoon (LWL), (2) discuss the methodology to model a seagrass rest oration target in LWL; and (3) discuss potential seagrass restoration targets for the three segments of the lagoon.

LWL has realized significant losses of seag rass over time due to water quality impacts, dredging, and urbanization. Seagrass loss and their retreat to shallow water have been attributed to a reduction of available light wh ich is confirmatory of the poor water quality in the lagoon. Mandate for the protection of seagrass comes from several sources including the 1998 LWL Management Plan, Pa lm Beach County Comprehensive Plan – Coastal Management Element, Florida Statut e 376.121 - Liability for Damage to Natural Resources, and the Federal Endangered Species Act protecting listed sp ecies of seagrass. Historical seagrass surveys from 1940, 1975, 1990 and 2001, and predictive modeling are presented in this report to provide a benchmark for the development of the seagrass restoration model.

Seagrass restoration target depths for each la goon segment at which adequate depth, light, and substrate conditions to ensure seagrass grow th and future recruitment were identified. Based on the GIS modeling condu cted, a possible LWL seagrass restoration target is 2,092 1 acres based on best available inform ation and understanding of the estuarine system. The possible target is based on 2001 seagrass distribution data and modeling of potential seagrass habitat which includes adequa te depth and substrate. This target may be achieved over a 20 year pe riod through the management of nutrient and sediment loadings, removal of muck sediments in the lagoon, and diversion of freshwater

1 2,092 acres = 1,646 acres existing + 466 acres of recruited seagrass2 discharges. Developing a seag rass restoration target or numeric goal based on available understanding of the history and physical char acteristics of an estuary sets up guidelines for ongoing and future habitat restoration projects.

LWL is located on the eastern shore of Sout h Florida in Palm Beach County (Figure 1) and is approximately 20 miles in length, 1⁄2 mile wide with average depth of 6 feet. It is estimated that current seagra ss coverage is 1,626 acres base d on an aerial assessment of 2001 conditions. The LWL is divided into thr ee segments (north, central, and south) based on hydrological factors including wate r quality, circulation, and physical characteristics

Figure 1. Location of Lake Worth Lagoon, Palm Beach County, Florida3 Table 1 presents the current distribution of seagrass in LWL by segment.

Table 1. Current Seagrass in Lake Worth Lagoon LWL Segment 2001 Seagrass (Acres) Segment Area (Acres) % Seagrass Cover North 1,134 3,386 33.5% Central 192 2,102 9% South 300 1,747 17% Totals 1,626 7,235 22%

Seagrass Background and Current Status

Seagrass beds are highly productive and ecolo gically important habitats within South Florida's estuaries and coastal lagoons. The combination of plentiful shelter and food results in seagrass beds being perhaps the richest nursery and feeding grounds in South Florida's coastal waters. Seagrass beds are nursery grounds for the juveniles of a variety of finfish and shellfish of commercial and recreational fi shing value. The roots and rhizomes stabilize the sediments, promote sedime ntation of particles, inhibit resuspension of sediments, and maintain an active envi ronment for nutrient recycling. The seagrass blades provide a substrate on which epiphytes can attach, providing a more valuable food source to most consumers. Seagrasses probably are more important to the food web as detritus than as a source for direct herbi vory. Detritus is an important food source for deposit feeders, providing polychaetes , amphipods, isopods, ophiuroids, some gastropods, and mullet with much of their nutrition. Seagrasses are also important because they are the primary food source fo r the Florida manatee (Palm Beach County Environmental Resources Mana gement (ERM) & FDEP 1998).

Losses of seagrasses have been documented worldwide and have been correlated with anthropogenic eutrophication and alteration of shorelines and habitats. Declines, as seagrass beds typically retreat to shallow de pth, have been correlate d with a reduction in available light due to an in crease in phytoplankton, epiphy tic or macroalgal growth (Bortone 2000). The LWL Management Plan recognizes the importan ce of this resource and the factors responsible for the decline of seagrass and has addr essed it through the goal of attaining and maintaining water quality and sediment quality of sufficient quality to sustain a healthy estuarine ecosystem.

The most recent assessment of seagrass cove rage was conducted in 2001. Results of the 2001 seagrass mapping project usin g aerial imagery demonstrate that seagrass beds cover at least 1,626 acres or approximately 22% of the lagoon (Baymont 2001). Additional seagrass was present; however, the lack of water clarity prevented mapping of all seagrass beds. The coverage varies throughout the three segments of the lagoon. Specifically, 69.7% of the seagrass in LWL ar e in the northern se gment of the lagoon, 11.8% of the seagrass are in the central segm ent of the lagoon, and 18.5% of the seagrass are in the southern segment of the lagoon. The overall seag rass coverage within each of the segments is 33.5% in the north, 9% in th e central, and 17% in the south segment of the lagoon.4 Methodology for Developing a Model

Step 1: Assembling Data Sets and Base Analysis The first step in developing the model is to assemble existing data sets and determine the distribution of seagrass by dept h using seagrass cove rage and bathymetry data. Figure 2 presents the intersection of bathymetry point da ta and seagrass data in a selected area of
LWL.

Figure 2. Intersection of Bathymetry Transect Data and Seagrass Data for Determination of Seagrass Distribution by Depth.

Approximately 92% of the seagrass in the north segment is present at depths of -2.0 to -6.0 feet NGVD. The average depth of seagrass in the north segment is -5.0 feet NGVD. Figure 3 presents the distributi on of existing seagrass by depth.

Figure 3. Distribution of Existing Seag rass by Depth in Lake Worth Lagoon North Based on Analysis of Aerial Photographs and Bathymetry.

LWL NORTH - DISTRIBUTION OF 2001 SEAGRASS

12 102 117 201 239

166

89 60 53 52 23 10 6 3 2 0 50 100 150 200 250 300

-1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -11 -12 -13 -14 -15

ELEVATION (FEET NGVD)ACRES Legend

Seagrass_Bathy
Elevation NGVD

-14.300000 - -10.80000 0

-10.799999 - -9.300000

-9.299999 - -8.000000

-7.999999 - -6.700000

-6.699999 - -5.600000

-5.599999 - -4.700000

-4.699999 - -3.800000

-3.799999 - -2.900000

-2.899999 - -2.000000

-1.999999 - -0.200000

2001 Sea g rass5 The central segment of LWL is the most se verely impacted segment of the lagoon with respect to water quality and the presence of mu ck. The central segment is currently listed on the FDEP Verified List of Impaired Wate rs. As water quality and light attenuation decrease, seagrasses have been known to retr eat to shallow depths (Bortone 2000). The central segment of the Lagoon typifies this ph enomenon. In a striking contrast to the north segment, there are only 192 acres (9% coverage) of seagra ss in the central segment. The majority of seagrass is only found at de pths between -3.0 to -5.0 feet NGVD. The average depth of seagrass in the central segm ent is -4.4 feet NGVD. Figure 4 presents the distribution of existing seagra ss in the central segment.

Figure 4. Distribution of Existing Seag rass by Depth in Lake Worth Lagoon Central Based on Analysis of Aerial Photographs and Bathymetry.

LWL CENTRAL - DISTRIBUTION OF SEAGRASS

0 9 45 48

31

19 16 13 7 3 1 1 0 10 20 30 40 50 60

-1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -11 -12

EKEVATION (Feet NGVD)ACRES

The south segment of LWL is also stressed due to poor water quality and is typified by the retreat of seagrass to shallow depths where light requirements can be met. Presently, there are only 300 acres (17% coverage) of se agrass in this segment. The majority of seagrass is only found at depths between - 3.0 to -4.0 feet NGVD. The average depth of seagrass in the south segment is only -3.4 f eet NGVD. Figure 5 pres ents the distribution of existing seagrass in the south segment.

Figure 5. Distribution of Existing Seag rass by Depth in Lake Worth Lagoon South Based on Analysis of Aerial Photographs and Bathymetry.

LWL SOUTH - DISTRIBUTION OF SEAGRASS

0 36 100

83

47

19 8 3 3
0 20 40 60 80 100 120

-1 -2 -3 -4 -5 -6 -7 -8 -9

ELEVATION (FEET NGVD)ACRES ELEVATION (FEET NGVD)6 Step 2: Digital Elevation Model The second step is to generate bathymetry for the en tire LWL based on a bathymetry transects performed in 2003 (Morgan & Eklund). Using 3D Analyst, the bathymetry data is interpolated to a raster data set using th e inverse distance weighted (IDW) interpolation technique. This technique is used to dete rmine cell values usi ng a linearly weighted combination of a set of sample points. The rast erized data set is reclassified as necessary for ease of analysis of other factors. Now that a digital elevation model for the LWL has been created, analysis based on depth can be achieved for a variety of factors. For example, different seagrass recruitment scenar ios may be set up based on depths at which seagrass is likely or targeted to recruit. Other ecosystem component models may be developed for other benthic organisms usi ng this approach (e.g., American oyster).

Step 3: Modeling Substrate While seagrass recruitment is dependent upon a complex set of factors and dynamics, there are basic building blocks which are impor tant to the colonization of seagrass. One of the building blocks is suitable substrate. For this exercise, substrate characteristics were modeled for LWL because a substrate ma p had not been previously developed. Historical data from the late 1960’s coll ected by the U.S. Environmental Protection Agency was digitized and combined with a muck survey perfor med during the 2003 bathymetric data collection event. In this case, muck deposits greater than 1 foot were mapped principally in the central and southe rn segments of LWL using a dual frequency fathometer. These data sets were integrated into a single shapefile. Once integrated, a Triangulated Irregular Network (TIN) was crea ted to interpolate unmapped areas of the LWL. The TIN was then converted to raster and reclassified. Figure 6 presents the raw data and reclassified substrate map.

Figure 6. Digitized Substrate Characteristics from Sediment Cores and Muck Survey (A) and Substrate Contour Map (B)

In the same manner that seagrass distribution by depth is analyzed, seagrass distribution by substrate is developed. Figure 7 presen ts seagrass distribution within LWL by substrate.

A B Legend

Substrate Contour

Shell-Pebble
Shelly Sand
Shelly Sand

Sand
Silty Sand
Silty Sand
Sandy Silt

Silt

Peat
Sediment

TYPE

SHELL GRAVEL PEBBL E

SHELLY SAND

SAND

SILTY SAND

SANDY SILT

SILT

PEAT7 Figure 7. Lake Worth Lagoon Modeled Substrate Characteristics Distribution

LWL Substrate Characteristics

Shell Pebble Gravel, 0.5%

Shelly Sand, 10.3% Silt, 0.7%

Peat, 0.6% Sandy Silt, 20.1%

Sand, 44.6% Silty Sand, 23.2%

Note: LWL Substrate Characteristics based on 1990 Lake Worth Lagoon Natural Resource Inventory and Morgan & Eklund 2003 Bathymetry/Muck Survey.

Figure 8 presents LWL 2001 seagrass da ta distribution by modeled substrate characteristics. An analysis of the results indicate that seagrass is predominantly found in sandy environments and does not colonize well in silty environments. However, further analysis of the results demonstrates that seag rass is colonizing silty environments within the central and southern segmen ts of LWL. These seagrass beds, found in impacted areas of LWL are not typical of healthy beds. Although seagrass may be found in these environments, seagrass is not expected to su ccessfully recruit in areas with high silt

content.

Figure 8. Lake Worth Lagoon Seagrass Distribution by Modeled Substrate Characteristics

Seagrass Distribution By Modeled Substrate Type

Shell Pebble Gravel, 0.1%

Shelly Sand, 9.7% Sand, 65.0% Silty Sand, 17.2%

Sandy Silt, 7.4%

Silt, 0.4%

Peat, 0.3%8 Step 4: Developing the Target

Several factors were analyzed to determine a possible targ et for seagrass restoration. These involve using a historical seagrass m odel, earlier seagrass mapping studies, and the use of current seagrass dist ribution, and substrate characteristics to predict future recruitment.

The earliest seagrass survey for LWL was performed in 1940 (Palm Beach County ERM & Florida DEP 1998) and found that 4,271 acres of seagrass were present. An analysis performed for this study using current seagra ss distribution data confirmed the results of the 1940 study. The comparison was developed based on the premise that given pristine water quality conditions in th e lagoon and ideal depth at wh ich light penetration allows for the growth of seagrass, a prediction can be made as to the total distribution of seagrass in an arbitrary year (e.g., 1940). Assuming that seagrass would be present at - 9.0 feet NGVD, the total amount of seagrass is predicted at 4,440 acres. The prediction is within 4% of that su rveyed in 1940. It should be noted that areas of the lagoon have been deepened since 1940 and this has not been accounted for in the current study.

In 1975, a resource inventory was performed and found that only 161 acres of seagrass remained in LWL, representing a substantia l loss of seagrass from 1940. The loss of seagrass was hypothesized to be linked to sewage disposal outfalls that directly discharged to LWL as well as degraded water quality. This was inevitable due to the rapidly expanding population surro unding the estuary and the t echnology associated with wastewater treatment plants. During the 1950s , an estimated ten million gallons per day (MGD) of raw sewage was discharged to LWL resulting in extensive bacterial and nutrient pollution. By 1970, seven major waste water treatment plants had been constructed, discharging 18.49 MGD of secondar ily treated sewage effluent. The volume was reduced to 2.98 MGD by 1984, largely as a result of the NPDES program administered by the USEPA (Palm Beach County ERM & Florida DEP 1998). Today there are only two small “package” wastewater treatment plants discharging into LWL.

In 1990, a natural resource in ventory was performed on LWL (Dames & Moore) and included detailed in-water surveys which provide d the most complete information to date. The survey indicated that there were 2,110 acres of seagrass; approximately half of the amount of seagrass in 1940. However, th ere was an increase of 1,945 acres when compared to results of the 1975 survey. It should be noted that the surveys used markedly different methodologies and the prece ding discussion is designed to illustrate magnitude of change.

In 2001, true color aerial photographs were interpreted to dete rmine the seagrass coverage in LWL. While this did not incl ude extensive in-water ground truthing, the goal was to develop a method to be used for a base line for future studies. The north segment was determined to have the majority of seagrass, 1,134 acres followed by the south segment with 300 acres; and the central segmen t with 192 acres of seagrass. The total coverage was determined to be 1,626 acres of seagrass. Although the methods of analysis were markedly different for the 1990 and 2001 surveys, there appears to be a loss of seagrass coverage over an 11-year peri od. One possible expl anation for the loss during the 1990’s is that development increa sed rapidly in the watershed causing upland disturbance and increased stormwater discha rge. This development led to increased9 eutrophication and release of sediments in to the lagoon which supported by water quality data for the time period. Table 2 presents a summary of seagrass coverage in LWL from 1940 through 2001.

Table 2. Historical Seagrass Coverage in Lake Worth Lagoon Year Seagrass (Acres) % Change 3

1940 4,271 - 1940 Model 1 4,440 2

- 1975 161 (96%) 1990 2,110 1,210% 2001 1,626 23%

1 Arbitrary date reflects conditions prior to intense ur banization; conditions allow for maximum coverage of

seagrass. 2 4,440 acres is the maximum of seagrass given pre-WWII conditions under these model assumptions.

3 Due to gross differences in survey methods, these values should only be used to indicate an order of magnitude change.

Target Development

To establish a seagrass restoration target fo r LWL, a benchmark time period is needed. Although the 1940 seagrass beds were modeled, th ere have been significant changes in shoreline and many areas that have been dre dged and filled since the 1940’s. Overlaying the 1990 or 2001 seagrass maps over the 1940 covera ge to determine restorable areas has limitations given the above changes to the system. Likewise, overlaying the 2001 maps over the 1990 map is also problematic due to the difference with which the surveys were conducted. Therefore, it is recommended that the 2001 seag rass survey be used as the benchmark time period supplemented by predictive modeling.

One goal of the management plan is to halt th e loss of current seagrass beds in LWL. A second goal is to restore seagrass to an historic and achievabl e level given the effects of a highly urbanized estuary and the effects of rapid population growth in Palm Beach County. A compromise needs to be achieve d between the 1940 coverage of 4,271 acres and the 2001 coverage of 1,626 acres of seagrass. A third goal not explored in this paper is to preserve existing seagrass beds with a focus on preserving high-density beds.

Rather than choosing an arbitrary date as a restoration target which may be unachievable in the long run, it may be re asonable to expect incremental improvements in water quality which in turn will allow more light to penetrate the water column. This small improvement in light attenuation will provide the environment for the restoration of seagrass throughout LWL.

Utilizing historical data is not without probl ems due to changes in technology and survey method differences. Therefore, using curr ent seagrass distribution to predict future seagrass distribution along with examining hi storical trends should result in setting a suitable target for seagrass restoration. As presented earlier, analyzing 2001 seagrass distribution in concert with 2003 bathymetri c data for LWL results in establishing seagrass distribution by depth (See figures 2-4). The average seagrass depth for each LWL segment was computed using seagrass di stribution and bathymetric data. Setting a10 target depth assumes that there should be at least a small improvement in water quality that would allow for increased light attenua tion through the water column. Using a very conservative approach of increasing the averag e seagrass depth by one foot in the north segment and 0.6 feet in the central and south segments, respectively, leads to the target depths as presented in Table 3.

Table 3. Average Seagrass Depth and Potentia l Target Depth by Segment in Lake Worth Lagoon Zone Average* Seagrass Depth Feet in NGVD Target Depth

North -5.0 -6.0
Central -4.4 -5.0
South -3.4 -4.0 *Weighted average

Predicting where seagrass is likely to recr uit based on depth and suitable substrate provides a more realistic targ et. Figure 9 presents a snap shot of where seagrass is predicted to recruit in conjunction with suitable sediment characteristics within 5 feet of the surface.

Figure 9. Snapshot of Suitable Depth and Substrate for Seagrass Recruitment

In summary, it is recommended to use seagrass depth distribution data and substrate data to develop segment specific s eagrass target depths, expres sed as NGVD. Potential target depths are: -6.0 feet for the north segment, - 5.0 feet for the central segment, and -4.0 feet for the south segment. These targets will allow for the protection of the existing 1,626 acres and the restoration of 466 acres of seagrass. Tabl e 4 presents the current distribution of seagrass in LWL as well as the restoration targ et for seagrass in each of Legend

Substrate Contour

Shell-Pebble
Shelly Sand
Shelly Sand

Sand
Silty Sand
Silty Sand
Sandy Silt

Silt

Peat
Potential Seagrass Habit a11 the three segments. The potential seagrass recruitment target is the sum of existing seagrass plus potential seagrass recruitment as predicted by both depth and substrate.

Table 4. 2001 Seagrass Acreage and Po tential Seagrass in Lake Worth Lagoon
LWL Segment 2001 Seagrass (Acres) Potential Seagrass by Depth (Acres) Potential Seagrass by Depth and Substrate Potential Seagrass

Target (Acres) Increase

North 1,134 321 197 1,331 17% Central 192 537 204 396 106% South 300 236 65 365 22% Totals 1,626 1,094 466 2,092 29%

It is evident from Table 4 that the greates t potential for seagrass recruitment is in the Central segment of LWL. There are extensive, relatively shallow areas that currently are not colonized by seagrass. Therefore, it woul d appear that efforts should be directed toward water quality and sediment manageme nt in this portion of the lagoon. A map of LWL with the target developed for this model is presented in the Appendix.

Achieving the Target

The LWL seagrass restoration goal will be acc omplished when the deep edges of the seagrass eventually extend to, delineated by high-resolution aerial photography, the target depths and substrate delineation set for each segment. Protection of existing coverage and the restoration of an additional 466 acres (based on model results) of seagrass may be accomplished through management of external nut rient and sediment loadings as well as the removal of muck sediments in the central segment of LWL. A possible timeframe for the proposed restoration target is 20 year s. These goals are outlined in the LWL Management Plan and are currently being implemented. For example, the LWL Partnership Grant Program has received $12,80 0,000 from the Florida State Legislature since 1999. A total of 37 construction projec ts have been funded as well as baseline monitoring projects. The following projects have been implemented to initiate the restoration of seagrass:

• 17 stormwater projects treati ng more 1,200 acres of runoff;

• 11 habitat enhancement projects cr eating new and restored habitat;

• 3 sewage treatment projects; and

• 7 baseline studies assessing the health of LWL

Additionally, PBC ERM has proa ctively embarked on many re storation and enhancement projects (Munyon Island, Peanut Island, Snook Islands, J ohn’s Island, and many mangrove planter projects) to in crease habitat in LWL.

The Comprehensive Everglades Restoration Plan (CERP) is addressing water quality, sediments, muck deposits and habitat in LW L through the Northern Estuaries Module. Additionally, the Palm Beach County Comprehensive Plan – Coastal Management Element addresses improving water qualit y in estuarine environments.1213 End Notes

There are several limitations and uncertainties associated with the possible restoration target. The target is based on a 2001 aeria l interpretation of seagrass which mapped seagrass beds of 0.25 acres or larger. The proposed target assumes that depth, light penetration, and substrate are the major fact ors affecting seagrass recruitment. Further study is recommended on assessing sediment characteristics of th e lagoon and potential seagrass recruitment. Additionally, this study does not take into account annual variations, effects of natural events (e.g., weather events), di version projects affecting the lagoon (e.g., C-51 canal diversion), or water qu ality of contributing basins to the LWL watershed. The possible seagra ss restoration target discussed in this paper was developed to foster support for improved conditions in LWL. The methodology and target are currently under review and ha ve not been adopted by the LWL Program or PBC ERM.

Paul Davis, Environmental Manager with P BC ERM, provided the conceptual framework and support for this study. Joyce Moody, GIS Analyst with PBC ERM, provided valuable technical support.

References

Baymont. 2001. 2001 Lake Worth Lagoon Seagrass Mapping Project . 70 pages.

Bortone, S.A. Seagrass: Monitoring, Ecology, _Physiology and Management . CRC Press, Boca Raton, Florida, USA.

Dames & Moore. 1990. Lake Worth Lagoon Na tural Resources Inventory and Resource Enhancement Study . 46 pages.

Federal Register/Vol. 63, N o. 177/Monday, Sept. 14, 1998, pp. 49035-49041.

National Marine Fisheries Service/National Oceanic and Atmospheric Administration. 2002. Final Recovery Plan for Johnson’s Seagrass. 110 pages.

Palm Beach County Planning, Zoning and Bu ilding Department. 1990. Revised 2003. Palm Beach County Comprehensive Plan

.
Palm Beach County Department of Environm ental Resources Management and State of Florida Department of Environmental Protecti on, Southeast District . 1998. Lake Worth Lagoon Management Plan . 257 pages.

Palm Beach County Department of Envir onmental Resources Management. 2004. Draft Lake Worth Lagoon Potential Seag rass Recruitment Scenarios Map . Unpublished.

Author Information

Rod A. Braun, Senior Environmental Analyst Palm Beach County Environmental Resources Management 2300 North Jog Road, 4 th Floor West Palm Beach, FL 3341114 Ph: 561-233-2536 Fx: 561-233-2414 rbraun@co.palm-beach.fl.us http://www.co.palm-beach.fl.us/erm/enhancement/lwlagoon.asp